When Work Notes and Additional comments are in different sections, the Additional Comments field has a yellow watermark like Work Notes and added content is duplicated in the Work Notes.
Configure the incident form layout and place Work Notes in the main Incident section and Additional Comments with Activities (filtered) in the Notes section.
Open an incident from the list whose status is not equal to closed.
Note that Additional Comments appears with the yellow watermark and adding content to it also updates the Work Notes field simultaneously.
Place Work Notes and Additional Comments in the same section of the form layout.